The centrifugal compressor used in a gas turbine engine or a jet engine typically includes an axial diffuser. In an axial diffuser, a plurality of axial diffuser passages are provided around the impeller at a regular circumferential interval, and as the radial flow of the fluid (compressed air) expelled from the impeller flows through each of the axial diffuser passages, the fluid flow is directed into an axial flow which is substantially in parallel with the central axial line of the impeller, and the kinetic energy of the fluid flow is converted into pressure energy. See U.S. Pat. No. 6,280,139, for instance.
Also is known the radial diffuser which is used in a centrifugal compressor to convert the kinetic energy of the fluid expelled from the impeller into pressure energy by reducing the velocity of the fluid in the radial flow. See Japanese patent laid open publication No. 2002-98093, for instance.
Each diffuser passage of an axial diffuser may consist of a tube member. If the tube member is straight, there is no centrifugal force, and, consequently, there is no unevenness in the fluid flow. However, in reality, because each diffuser tube is highly curved so as to direct the direction of the fluid flow from the tangential direction of the impeller into the axial direction, a centrifugal force is produced, and some pressure gradient (in the cross section of the diffuser tube) is produced in the fluid flowing in the diffuser tube.
In the case of the conventional axial diffuser using diffuser passages each having an elliptic cross section, no consideration was made to balance the centrifugal force along the major axis of the elliptic cross section of each diffuser passage. For this reason, a pressure unevenness or gradient in the cross section is produced from the first bend of each diffuser passage. In particular, a low velocity region (low momentum flow at the boundary layer) builds up on the negative pressure side of the diffuser passage so that the fluid velocity tends to be lower on the negative pressure side of the downstream end of the diffuser passage. Conversely, the fluid velocity is higher on the positive pressure side of the downstream end of the diffuser passage. Therefore, a significant pressure gradient is produced between the positive pressure side and negative pressure side at the downstream end of the diffuser passage.
Furthermore, in the negative pressure side, owing to a centrifugal force (inertia force) produced in the bend of the passage, a part of the fluid flow may separate from the wall surface, and this may create vortices. As a result, a part of the kinetic energy of the fluid flow is dissipated as heat, and the vortices diminish the effective cross sectional area of the fluid passage by blocking the fluid flow. For these reasons, the fluid velocity may not be reduced as designed. Therefore, in a conventional axial diffuser, the static pressure recovery ratio Cp=(static pressure at diffuser outlet−static pressure at diffuser inlet)/(total pressure at diffuser inlet−static pressure at diffuser inlet) is not so high as desired, and this prevented the effective efficiency of a centrifugal compressor to be increased to a desired level.